If you are willing to home cook for Roxy, I would suggest you do a phone consult with Dr. Remillard at MSPCA/Angell Memorial Hospital in Boston. She has a special interest in kidney disorders and is currently doing a study in this area. The website for the information is
Nutrition Services
She also can advise you in consult with acceptable choices for commercial foods - you don't want to just pick one off the shelves in this situation.
Best wishes.